From The Public Domain Movie Database:
A young boy is playing in the yard with some toy trains under the watchful eye of his pet dog. A real train rolls by and he runs off to get a closer look. The dog breaks its rope and gives chase after the young boy who has managed to climb aboard the now motionless train. The boy has started the train and is headed towards a collision with another train. The dog saves the boy in the last second before the trains collide.
Rather odd, and somewhat scary cartoon here about a child who dreams about being a train engineer. Luckily, a train track is LITERALLY outside their back door (yikes!) and LUCKILY the family has a Saint Bernard to keep an eye on the tyke. The Tyke smarts out the Dog though, and sneaks out and boards the train when he's not looking! After the kid falls off and HITS HIS HEAD ON THE TRAIN TRACK (Ow!) the kid has a dream / nightmare where the train gears have come to life and proceed to dissapear when the train the kid is riding on, is coming to a collision with another train! Bonus points for the other train screaming like Homer Simpson. Anyways, the cartoon ends with an "Dog saves the day" romp. Pretty wild (and pretty french?)